Study Of Molecular Resistance Mechanism And Epidemiology Of Carbapenem-resistant Acinetobacter Baumannii

Objectives To investigate antimicrobial resistance in Acinetobacterspecies from 7 teaching hospitals in China; To investigate molecular epidemiology of carbapenem-resistant Acinetobacter species; To study the molecular mechanism of resistance to carbepenems; To determine the role ofβ-lactamases and integrons inmediating the resistance to multiple drugs orpan-drugs among Acinetobacter baumannii. Methods Non-repetitive 145meropenem susceptible(19) and non-susceptible (126) Acinetobacter baumanii clinical isolates were collected in Severn teaching hospitals in 2005．Minimum inhibitory concentrations (MICs) of meropenem, imipenem and others antibiotics were determined by agar dilution and interpreted using Clinical Laboratory Standards Institute (CLSI) breakpoints. The homology of the isolates was determined by both pulsed field gel electrophoresis (PFGE) and repetitive extragenic palindromic-polymerasechain reaction(REP-PCR) . blaSHV, b1aVEB, b1aGES, b1aIMP-, b1aVIM-, b1aOXA-23-like, b1aOXA-24-like,b1aOXA-58-like, b1aOXA-51-like genes for these strains were amplified and sequenced. For class 1 integrons, twelve representative clones were amplified and sequenced. Results Meropenem non-susceptible strainsresistant rate of cefoperazone/sulbactam, piperacillin/tazobactam, ceftazidime, cefepime 81．7% to 100%, ciprofloxacin and amikacin ranged from 93．1% to 88．1%, minocycline 67．5% but the susceptible rate of polymyxin B 100%. Molecular epidemiology of Meropenem non-susceptible strains typing by PFGE and Rep-PCR showed that six different clones were identified in seven different large teaching hospitals. P clone occurred at Peking Union Medical College Hospital(PUMCH), Beijing 304 hospital(304H) and Beijing Hospital (BJH). PFGE pattern A were also found at four hospitals, including PUMCH, Dalian No.1 Medical College Hospital(DLH), Zhejiang University No. 1 Medical College Hospital(ZJH) and Nanjing Provincial Pepole Hospital (NJH). In addition to P and A clone, other 2 different clones have been found at PUMCH named L clone(1 case) and G clone(4 cases). T clone was an nother epidemic clone in eight CRAB at Zhejiang hospital. F clone was identified in Beijing Chaoyao hospital. The multiplex PCR assay amplified fragments ofb1aOXA alleles encoding each of the four subgroups of OXA carbapenemases inAcinetobacter spp. Genes encoding b1aOXA-51-like were detected in 136 of 145(93．8%) isolates, b1aOXA-23-like with b1aOXA-51-like in 65 (51．6%) isolates,b1aOXA-23-like 8 isolates and b1aOXA-24-like 1 isolate from 126 non-susceptible isolates, respectively. Alleles encoding OXA-58-1 ike enzyme were detected (together with b1aOXA-23-like and b1aOXA-51-like) in 3 isolates from a single hospital. Sequence analysis of the products from different hospital and MICs for the b1aOXA-51-like amplicons (13cases) identified b1aOXA-66 (12 cases) andb1aOXA-68 (1case) , gene encoding b1aOXA-58-like identified b1aOXA-58, b1aOXA-24-like was b1aOXA-72, whereas b1aOXA-23-like (15 cases) sequences wereclassical b1aOXA-23 . The genes of b1aVIM, b1aIMP,b1aGES and b1a_VEB were notdetected in all of 145 isolates in this study. Seven kinds of class 1 integrons were found in 12 representive clones. Five integrons contained the genes conferring resistance to aminoglycosides, such as aacC1, aadA1, aacA4, aacA4 and aacA6．Two integrons contained the gene which conferred resistant to rifampin encoding a rifampin ADP-ribosylating transferase (arr3) , two integrons had the gene encoding a chloramphenicol acetyltransferases (catB6) .Conclusions Outbreaks of Meropenem non-susceptible clone occurred at 7large teaching hospitals. Different clones can be found among different hospitals. This study showed that Meropenem non-susceptible clone spread was the main reason for the increasing carbapenem resistance trend at different hospitals. Epidemic carbapenem resistant clones were different among the hospitals. The most prevalent carbapenemase was OXA-23 enzyme. These findings indicated wide dissemination of OXA-66/OXA-51-like type of carbapenemase and present in almost all A. baumannii isolates. ClassⅠintegrons with different resistance gene cassetes played an important role in the multi-drug resistance of this organism. Carbapenem-resistant Acinetobacter species was susceptible to uncommon used antimicrobials agents, such as colistin and minocycline.